1. Field of the Invention
This invention relates to a catalytic apparatus which removes volatile organic compounds from a gas stream such as air and a method of removing such compounds. The apparatus and method particularly involves heating the gas stream during its flow through the apparatus to effectively produce a catalytic reaction of the volatile organic compounds prior to exhausting the latter to the atmosphere.
2. Description of the Prior Art
Volatile organic compounds include a class of contaminants such as hydrocarbons, ammonia, SO.sub.x and the like which are regarded as harmful if released into the atmosphere. When discharged into the atmosphere without treatment, these compounds react with NO.sub.x and sunlight and produce, inter alia, ozone (O.sub.3) and harmful pollutants. Ozone is regarded as a contaminant in the lower atmosphere and is thought to contribute to the development of smog. As a result, the release of volatile organic compounds is being increasingly regulated by federal and state agencies charged with environmental protection.
Volatile organic compounds (V.O.C.s) are released from a variety of different everyday products and chemicals. One such example is industrial finishes, e.g., solvent based paint. When paint is applied, or in the process of curing and drying of paint, V.O.C.s are released by evaporation. Thus, the location at which the paint is applied may be regarded as a pollution point source, and subject to governmental environmental controls. Often, the V.O.C.s can be applied, dried or cured in an oven or other substantially enclosed area where the V.O.C.s can be entrained in a gas stream for capture or conversion to acceptable gases or vapors such as CO.sub.2 or H.sub.2 O. For example, solvent-based paints are often dried in what is effectively a convection oven where warm or hot air is blown in, heated, and then exhausted. However, such convective ovens often present a problem in that large volumes (10-50 thousand ft..sup.3 /min.) of gas (e.g. air) must be treated which have only dilute concentrations of V.O.C.s. Cleaning up the exhaust air from such ovens presents a problem because of the backpressure build-up. It is far preferable to treat a lower volume (less than ten thousand ft..sup.3 /min.) of gas with a higher concentration of V.O.C.s and thus economize on the treatment apparatus and its cost of operation.
Prior art systems have been developed for catalytic destruction of pollutants from gaseous effluent. U.S. Pat. No. 2,013,979 to Bray discloses an apparatus for starting catalytic reactions including a tube from which a controlled amount of gas is allowed to escape. The gas is ignited and burns at a predetermined distance from a bed of catalytic material. The surface layer of the catalyst is heated by the flame, and when it reaches a kindling temperature, the catalytic reaction becomes self-sustaining. U.S. Pat. No. 3,467,491 to Hardison is directed to a catalytic system for removing ammonia from vent gases. The method disclosed in this patent includes the use of a heater coil placed within the inlet section of a reactor such that the vent gases passing through the vent will be preheated to a temperature on the order of 400.degree. F. prior to entering a shell portion of the reactor which contains a catalyst bed.
U.S. Pat. No. 4,213,947 to Fremont et al. discloses an emission control system where a burner is employed to heat a effluent gas prior to its entry into a catalyst zone. The burner serves to adjust the temperature of the effluent gas stream to the temperature at which it will be most effectively catalytically oxidized in the catalyst zone. U.S. Pat. No. 4,957,710 to Nagai discloses a catalytic exhaust gas processing device having a series of different catalyst layers. The purpose of the device is to render noxious gases into non-toxic and odorless gaseous carbon dioxide and water. In one embodiment of this device, a burner is used to heat a catalyst layer to the catalytic reaction temperature.
However, there has developed a need for a method and apparatus which effectively treats gas streams containing V.O.C.s to substantially eliminate or reduce to an acceptable level the concentration of V.O.C.s therein without subjecting the effluent stream to unacceptable backpressure. In addition, there is a need for an alternative method of heating the catalytic member which simultaneously destroys V.O.C.s within the reaction chamber. Because the destruction of V.O.C.s represents an added cost for the user, the apparatus and method to be used should not only be safe and effective, but should also be affordable to obtain and relatively inexpensive to operate.